Merge pull request #71 from ipld/rvagg/calculateHeaderLength

feat: add precise calculateHeaderLength()

lib/buffer-writer.js

@@ -1,4 +1,6 @@
 import varint from 'varint'
+import { Token, Type } from 'cborg'
+import { tokensToLength } from 'cborg/length'
 import * as CBOR from '@ipld/dag-cbor'
 
 // Number of bytes required without any roots.
@@ -250,6 +252,28 @@ const totalByteLength = (cids) => {
   return total
 }
 
+const headerPreludeTokens = [
+  new Token(Type.map, 2),
+  new Token(Type.string, 'version'),
+  new Token(Type.uint, 1),
+  new Token(Type.string, 'roots')
+]
+
+/**
+ * @param {number[]} rootLengths
+ * @returns {number}
+ */
+export const calculateHeaderLength = (rootLengths) => {
+  const tokens = [...headerPreludeTokens]
+  tokens.push(new Token(Type.array, rootLengths.length))
+  for (const rootLength of rootLengths) {
+    tokens.push(new Token(Type.tag, 42))
+    tokens.push(new Token(Type.bytes, { length: rootLength + 1 }))
+  }
+  const length = tokensToLength(tokens) // no options needed here because we have simple tokens
+  return varint.encodingLength(length) + length
+}
+
 /**
  * Creates synchronous CAR writer that can be used to encode blocks into a given
  * buffer. Optionally you could pass `byteOffset` and `byteLength` to specify a

package.json

@@ -61,6 +61,7 @@
   },
   "dependencies": {
     "@ipld/dag-cbor": "^7.0.0",
+    "cborg": "^1.9.0",
     "multiformats": "^9.5.4",
     "varint": "^6.0.0"
   },

test/test-buffer-writer.js

@@ -19,6 +19,11 @@ describe('CarBufferWriter', () => {
         const roots = new Array(count).fill(cid)
         assert.deepEqual(CarBufferWriter.estimateHeaderSize(count), createHeader(roots).byteLength)
       })
+      it(`calculateHeaderLength(${count})`, () => {
+        const roots = new Array(count).fill(cid)
+        const rootLengths = roots.map((c) => c.bytes.byteLength)
+        assert.deepEqual(CarBufferWriter.calculateHeaderLength(rootLengths), createHeader(roots).byteLength)
+      })
     }
     it('estimate on large CIDs', () => {
       const largeCID = CID.parse(`bafkqbbac${'a'.repeat(416)}`)